1. Field of the Invention
This invention relates to a method of recognizing an irradiation field in the case where a radiation image is recorded on a recording medium such as a stimulable phosphor sheet by limitation of the irradiation field of a radiation. This invention also relates to a method of adjusting image processing conditions by utilization of the method of recognizing an irradiation field in the course of processing of image signals detected from the recording medium.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhbiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed that a stimulable phosphor in a radiation image recording and reproducing system be used. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to electric image signals, image processing is carried out on the electric image signals, and the radiation image of the object is reproduced as a visible image by use of the processed image signals on a recording material such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing system, in order to improve the image quality, particularly the diagnostic efficiency and accuracy, of the visible image, image read-out for photoelectrically detecting the light emitted by the stimulable phosphor sheet upon stimulation thereof should preferably be carried out by use of read-out conditions adjusted to appropriate values in accordance with each radiation image. From this viewpoint, one embodiment of the aforesaid radiation image recording and reproducing system has been proposed in, for example, Japanese Unexamined Patent Publication No. 58(1983)-67240. In the proposed embodiment, before final read-out is carried out by scanning the stimulable phosphor sheet carrying a radiation image of an object stored thereon by stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the radiation energy stored, detecting the emitted light by a photoelectric read-out means and converting it into electric image signals, preliminary read-out for approximately detecting the image information stored on the stimulable phosphor sheet is carried out by use of stimulating rays of a level lower than the level of the stimulating rays used in the final read-out. Read-out conditions for the final read-out are adjusted on the basis of the image signals obtained by the preliminary read-out, and the final read-out is carried out by use of the read-out conditions. The electric image signals obtained by the final read-out are sent to an image processing means and are processed in accordance with the image recording portion of the object and/or the image recording method to obtain a visible image suitable for viewing, particularly for diagnostic purposes. The processed image signals are used to reproduce the visible image on a photographic film or the like.
By the term "read-out conditions" as used herein are meant various conditions affecting the relationship between the input to the read-out means and the output thereof, for example, the relationship between the input to the photoelectric read-out means (i.e. the amount of the light emitted by the stimulable phosphor sheet) and the output thereof (i.e. the level of the electric image signal) in the aforesaid case. For example, the term "read-out conditions" means the read-out gain (sensitivity) determining the relationship between the input and the output of the read-out means, the scale factor (latitude), and the power of the stimulating rays used for read-out.
As mentioned above, the level of the stimulating rays used in the preliminary read-out is adjusted to be lower than the level of the stimulating rays used in the final read-out. That is, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out is adjusted to be lower than the effective energy of the stimulating rays used in the final read-out.
In the case where the image information stored on the stimulable phosphor sheet is approximately ascertained prior to the final read-out and the final read-out is carried out by use of the read-out conditions adjusted on the basis of the image information, it becomes possible to eliminate adverse effects of a fluctuation in the level of the radiation energy stored on the stimulable phosphor sheet, which is caused by a change in the object or the image recording portion thereof, by a fluctuation in the radiation dose, or the like, and to obtain a visible image suitable for viewing, particularly for diagnostic purposes.
On the other hand, in the case where the aforesaid radiation image recording and reproducing system is used for medical diagnosis, portions of the human body not related to diagnosis should not be exposed to radiation since the radiation is harmful to the human body. Further, when the human body portions not related to diagnosis are exposed to radiation, the radiation is scattered by such portions to the portion related to the diagnosis, and the contrast and resolution are adversely affected by the scattered radiation. Therefore, in the aforesaid radiation image recording and reproducing system, the irradiation field is often limited in the course of the recording of a radiation image.
In the case where image recording is carried out by limiting the irradiation field as mentioned above, a portion inside of the irradiation field and a portion outside of the irradiation field are present on a single stimulable phosphor sheet. In this case, it is advantageous for the portion inside of the irradiation field, i.e. the position of the irradiation field contour, to be distinguishable.
This is because, in the case where the preliminary read-out is carried out on the stimulable phosphor sheet and the read-out conditions for the final read-out are to be adjusted on the basis, for example, of the image signals obtained by the preliminary read-out, the read-out conditions should preferably be adjusted on the basis of only the preliminary read-out image signals inside of the irradiation field on the stimulable phosphor sheet when image recording has been carried out by limitation of the irradiation field.
The aforesaid reason will hereinbelow be described in detail. As for the method of adjusting the read-out conditions for the final read-out on the basis of the image signals obtained by the preliminary read-out, a novel method has been proposed in, for example, Japaneses Unexamined Patent Publication No. 60(1985)-156055. The proposed method comprises the steps of: determining a histogram of levels of the image signals obtained by the preliminary read-out, calculating the maximum image signal level Pmax and the minimum image signal level Pmin of a desired image signal range in the histogram, and adjusting the read-out conditions for the final read-out so that the maximum image signal level Pmax and the minimum image signal level Pmin correspond respectively to the maximum signal level Qmax and the minimum signal level Qmin of a desired input signal range in an image processing means which are determined by the maximum density Dmax and the minimum density Dmin of a correct density range in the reproduced visible image.
However, in the case where image recording is carried out by limitation of the irradiation field as mentioned above, radiation scattered by the object within the irradiation field normally passes outside of the irradiation field. The scattered radiation is absorbed and stored on the stimulable phosphor sheet which exhibits high sensitivity, and therefore the histogram of the levels of the image signals obtained by the preliminary read-out includes the image signal level caused by the scattered radiation. Since the image signal level caused by the scattered radiation outside of the irradiation field on the stimulable phosphor sheet is often higher than the image signal level inside of the irradiation field, it is not always possible to discriminate between the image signal levels inside and outside of the irradiation field in the histogram obtained by the preliminary read-out. Therefore, in the case where Pmax and Pmin are calculated from the histogram as mentioned above and the read-out conditions are adjusted on the basis of Pmax and Pmin, the minimum image signal level inside of the irradiation field is not detected as Pmin, and that caused by the scattered radiation outside of the irradiation field is detected as Pmin. In general, the minimum image signal level outside of the irradiation field is lower than that inside of the irradiation field. Accordingly, when the minimum image signal level outside of the irradiation field is detected as Pmin, signals caused by the scattered radiation not related to diagnosis are taken within a low density range in the final read-out, and the density of the image of the portion related to diagnosis becomes too high. As a result, the image contrast decreases and it becomes difficult to make an accurate diagnosis.
More specifically, in the case where a radiation image is recorded by limiting the irradiation field, radiation scattered by the object passes outside of the irradiation field on the stimulable phosphor sheet and causes noise in the image signals obtained by the preliminary read-out. Therefore, when the read-out conditions are adjusted based on such preliminary read-out image signals, it is not always possible to adjust the read-out conditions to appropriate values and to obtain a visible image suitable for viewing, particularly for diagnostic purposes.
Therefore, in the case where radiation image recording is carried out by limiting the irradiation field and the read-out conditions are to be adjusted based on the preliminary read-out image signals by use of the aforesaid method, the irradiation field should be recognized accurately and the read-out conditions should be adjusted based on the preliminary read-out image signals inside of the irradiation field, thereby to eliminate adverse effects of the scattered radiation outside of the irradiation field.
Besides the case wherein the read-out conditions are to be adjusted for a radiation image stored on the stimulable phosphor sheet, recognition of the irradiation field is also necessary for various purposes in the case where a radiation image is recorded on a recording medium by limitation of the irradiation field.
On the other hand, in the aforesaid radiation image recording and reproducing system, image processing is carried ut as mentioned above on the image signals detected by image read-out. In general, the image processing is carried out by use of image processing conditions adjusted for each image in accordance with the image recording portion of the object and/or the image recording method to obtain a visible image suitable for viewing, particularly for diagnostic purposes. However, it may be considered preferable to adjust the image processing conditions on the basis of the image signals obtained, for example, by the preliminary read-out or the final read-out, instead of the image recording portion of the object and/or the image recording method, or on the basis of both the image signals obtained by the preliminary read-out or the final read-out and the image recording portion of the object and/or the image recording method.
However, as mentioned above, in the case where image recording is carried out by limiting the irradiation field, information (noise) caused by the scattered radiation outside of the irradiation field is contained in the image signals obtained by the preliminary read-out or the final read-out. Therefore, in this case, it is not always possible to obtain such appropriate image processing conditions as originally expected by the adjustment based on the image signals, even though the image processing conditions are adjusted on the basis of the image signals obtained by the preliminary read-out or the final read-out.
Therefore, in the case where image recording is carried out by limiting the irradiation field and the image processing conditions are to be adjusted based on the image signals as mentioned above, the image processing conditions should preferably be adjusted based on noise-free image signals obtained by eliminating the information caused by the scattered radiation, instead of being adjusted directly based on the detected image signals.
Besides the case of image recording utilizing the stimulable phosphor sheet, the aforesaid problems with regard to the adjustment of the image processing conditions arise generally in the case where a radiation image is recorded on a recording medium by limitation of the irradiation field.
By the term "image processing conditions" as used herein are meant various conditions affecting the relationship between the input to the image processing means and the output thereof, for example gradation processing conditions and frequency response processing conditions.